Electric motors and generators have certain characteristics rendering them suitable for use in many industries and devices for converting electrical energy into mechanical energy and vice versa. Despite being simple and reliable, electric machines have drawbacks.
One drawback is that the efficiency of electric machines drops dramatically when the load is below 70-50% of a machine's full load. This drawback is sometimes disregarded, since in general the efficiency of electric machines is much higher than of internal combustion engines and many other types of engines.
Another widely known drawback is that electric motors have higher power consumption during acceleration than in a steady state.
Multiple electric machine arrangements have been developed in an effort to address these limitations.
See for example, U.S. Pat. No. 4,525,655 that describes an electric motor drive system having a first direct current electric motor, a first drive shaft to which the first electric motor is coupled, a second direct current electric motor having a higher power rating than the first, a second drive shaft to which the second electric motor is coupled, the second drive shaft comprising a hollow sleeve with the first drive shaft extending there through, an epicyclic gear system comprising a sun gear to which the first drive shaft is coupled, a ring gear to which the second drive shaft is coupled, and planetary gears meshing with both the sun gear and the ring gear, an output drive coupled to the planetary gears, and, a one way rotation restraining means coupled to the ring gear to permit rotation of the ring gear in one direction and to prevent rotation of the ring gear in the other direction. This system is somewhat limited because the first and the second electric machines are coupled together at all times by an epicyclic gear system, and at low loads higher power rating machine is engaged too, along with the low power electric machine, working at a low efficiency point and therefore reducing the overall system efficiency. Even if it is possible to stop completely one of the machines while other is working (restraining rotation of one of the members of planetary gear set), such condition would change transmission ratio of the planetary gear set, forcing the second machine to operate at lower efficiency bandwidth. Independent and controllable contribution of each of the electric machines to the total performance of the device is not disclosed.
Another area of use of multiple electric machine arrangement is hybrid power trains, which combine multiple electric machines and an internal combustion engine (ICE). Since typically ICE's efficiency is much lower than efficiency of electric machines, the combination of both devices can increase the overall system efficiency.
Hybrid power trains typically consist of multiple electric machines interconnected together, an ICE, battery bank to store and provide electrical energy, and controllers. In such arrangements electric machines can perform both as motors, supplying power to the output shaft, or generators, converting ICE power into electrical one for battery bank charging.
U.S. Pat. No. 6,131,680 describes such system. In this invention crankshaft of ICE is mechanically linked to the planetary carrier, the first motor with the sun gear and the second motor with the ring gear. A controller drives the second motor with electric power regenerated by the first motor or drives the first motor with electric power regenerated by the second motor, based on a gear ratio of the sun gear to the ring gear of the planetary gear, thereby enabling the power output from the engine to be converted to a desired power and output to a power transmission gear mechanically connected with the ring gear. This structure allows the engine to be driven at a desired driving point of highest possible efficiency, thereby enhancing the efficiency of the whole apparatus. Even if it is possible to operate both electric machines as motors only or as generators only, the electric machines are coupled together mechanically through the planetary gear set and both electric machines are always engaged, limiting the efficiency of such system at low or intermittent loads. Independent and controllable contribution of each of the electric machines to the total performance of the device is not disclosed.
U.S. Pat. No. 6,962,545 discloses another hybrid power train. A parallel-hybrid transmission has one or more electrical motor/generator units. The motor/generators are coaxially or concentrically arranged with the transmission input and output shafts via planetary gear sets. Associated clutch closures selectively couple power to and from the sun gears and planetary carriers of the gear sets through a complex planetary gear configuration such as Ravigneaux gear set for switching between certain operational modes. The clutch and brake operations selectively achieve multiple ratio range operations including engine starting under electric power, high torque acceleration from a standstill, regenerative deceleration (braking), multiple ratio range operation, load sharing, rotation matched speed-less shifting and combined or individual continuously variable combustion engine and electric motor and/or generation modes.
This invention is limited by several factors. It uses complex system of clutches to switch between different modes of operation. In addition, since a system of clutches used, it does not allow a smooth, efficient and seamless combination or distribution of power between both electric machines. Moreover, it does not allow independent and controllable power combination or distribution between electric machines. Another limitation of the invention is the usage of complex Ravigneaux gear set as one of the gear sets, which adds to the complexity of the machine and reduces its overall efficiency.
U.S. Pat. No. 7,053,566 discloses yet another hybrid drive train. A drive train for a hybrid electric vehicle has an engine, first and second motor/generators, a third motor, and a transmission in connecting relationships with the engine, the motor/generators, and the third motor. The transmission has planetary gear sets to be shifted among a plurality of running modes including a large driving force running mode. A controller controls surplus power caused by power balance between the first and the second motor/generator to be supplied to the third motor when the surplus power is generated and the vehicle starts with the transmission being operated in the large driving force running mode. Since either both of the first and second motors/generators are either engaged together or have one of the motors/generators grounded using a clutch and/or a brake, the invention does not allow smooth and seamless combination of both electric motors/generators. Moreover, the usage of plurality of clutches and brakes adds to the complexity of the system. Further, the invention does not allow independent and controllable power combination or distribution between electric machines.
U.S. Pat. No. 7,371,201 discloses a family of transmission devices, consisting of three motors/generators. The three motors/generators are operated in a coordinated fashion to yield continuously variable forward and reverse speed ratios between the input shaft and the output shaft, while minimizing the rotational speeds of the motor-generators and optimizing the overall efficiency of the system. This invention is limited by the fact that all planetary gear sets are interconnected, therefore all of the motors/generators are also interconnected in such a manner that it is not possible to combine or distribute power completely independently between the electric machines at any given input to output speed ratio of the transmission system. Operation speed change of any of the motors/generators affects the operation speed of the rest of motors/generators at a given input to output transmission ratio.
The publication “Calculation of fuel consumption on hybrid power system driven by two motors connected in series by CVT” (Koji SAKOTA, Kazuya OKUBO, Toru FUJII, Proceedings of the international multi-conference of engineers and computer scientists. March 2010) describes a system, which consists of two motors: main motor and a sub motor, connected in series through a single continuously variable transmission (CVT) and a mechanical clutch. ICE is connected in series to the sub motor through a clutch as well. The sub motor is held at constant back electromotive force (EMF), while the main motor is connected to the vehicle's wheels and supplies power as it is demanded by driver. The role of CVT is to match the sub motors' speed to the speed of the main motor, since the sub motor works at constant, predetermined EMF. It was found during research that this set up increases efficiency of the entire drive train by approximately 6% on urban standard driving cycle, while in other conditions there was no efficiency increase. One of the objectives of the research was finding most beneficial predetermined and constant point of operation of the sub motor and most beneficial in terms of overall system efficiency rated output powers of the main and sub motors. One shortcoming of this proposed solution is its inflexibility. Since the motors are connected in series, they are coupled together so that the main motor works at all times, even while its working point is in a very inefficient bandwidth. Although the sub motor works constantly at predetermined working point with relatively high efficiency, its contribution to the overall system is limited since the main motor is always connected to the wheels and operates at all times, thereby reducing the overall system efficiency.
U.S. Pat. No. 6,637,283 discloses an apparatus for extending the drive ratios and versatility of a continuously variable transmission (CVT) includes an enclosure which houses parallel input and output shafts journaled to front and rear walls of the enclosure. The input shaft receives rotative force directly from one or two CVT units. Low gear, drive gear and reverse gears are disposed upon the input shaft. Corresponding driven low gear, drive gear and reverse gears are disposed upon the output shaft. The corresponding gears are connected by positive drive belts. Coupling devices axially sliding upon splines in the input shaft in response to a shifting lever achieve low, drive, reverse and park modalities of the apparatus. This invention cannot be used with electric motors or generators since it contains a relatively large number of gears and drive belts, which reduce significantly the efficiency of the system. In addition, the invention is of little value to electric motors since other more efficient techniques for implementing reverse rotation in electric motors have been provided. Further, the relatively high speeds and output torques of electric motors limits the applicability of the invention to electric motors.
As can be seen from the foregoing, there is a need for an electric machine which addresses the abovementioned shortcomings.